Use of the renewable source cellulose to produce liquid fuels and chemical products can effectively relieve the problem that fossil fuels, such as petroleum, are becoming exhausted day by day. Use of plant cellulose material to produce liquid fuels and chemical products is to hydrolyze the cellulose into carbohydrates, such as oligosaccharides and monosaccharides, and then ferment the carbohydrates to produce liquid fuels and chemical products with microbe. The common methods for hydrolyzing the cellulose into the carbohydrates mainly include diluted hydrochloric acid hydrolysis method, concentrated sulfuric acid hydrolysis method, and enzyme hydrolysis method. The concentrated sulfuric acid hydrolysis method has advantages of low reaction temperature, high yield, little by-product, etc., and a shortcoming of high acid recovery cost.
The concentrated acid hydrolysis process of cellulose includes two stages: {circle around (1)} one is main hydrolysis process of hydrolyzing the cellulose into carbohydrates (namely oligosaccharides) in concentrated acid to obtain main hydrolysate; {circle around (2)} the other is post hydrolysis process of hydrolyzing the carbohydrates into glucose in dilute acid to obtain post hydrolysate.
The commonly used method of recovering sulfuric acid from the concentrated acid hydrolysate of plant cellulose material is electrodialysis method. However, this method is capital intensive and high in running cost. Then, new methods of recovering acid have been gradually developed, such as the process described in U.S. Pat. No. 5,562,777 of acid hydrolysis of celluloses and hemicelluloses materials: mixing the plant cellulose materials with a solution of 70˜77% sulfuric acid by weight, keeping the reaction temperature in the range of 50° C.˜80° C., adding water to dilute the acid to a concentration of 20˜30% by weight, heating to a temperature of 80° C.˜100° C. at atmospheric pressure, and hydrolyzing for 40˜480 min, then separating the liquid portion from the solid portion, using strong acid cation exchange resins to adsorb sugars in the liquid portion and recover the acid, thereby obtaining 15% sugar by weight and 15% sulfuric acid by weight, finally evaporating water from the recovered dilute sulfuric acid to concentrate the sulfuric acid to a concentration of 70˜77% by weight for reuse. However, because of the low adsorption capacity (2 meq/g) of the cation exchange resin, it is usually applicable to absorb and separate a minute amount impurities from a large amount production, or absorb and purity a small amount expensive production, for example the purification of amino acid or enzyme. While the sugars to be separated from the biomass hydrolysate are a large amount, easily reaching to tens of thousands ton to millions tons per year, therefore the cation exchange resin is incompetent obviously. what's more, the absorptive and elution process of the cation exchange resin is very slow. To attain such yield, it is undoubtedly need a large amount of cation exchange resin, and the complex composition in the biomass hydrolysate easily makes the cation exchange resin poisoned and invalid. Thus the process in fact is not practical.
U.S. Pat. No. 4,608,245 discloses a method of recovering the sulfuric acid by extraction comprising: mixing cellulose-containing materials with 70˜72% sulfuric acid by weight for 10 min at 50° C.; keeping the radio of sulfuric acid to cellulose greater than 7.2; adding water to dilute the acid to a concentration of 40˜50% by weight; keeping temperature at 90° C. for 2 min; separating the liquid portion from the solid portion to obtain lignin and hydrolysate; extracting the cooled hydrolysate for the first time with C4˜C7 alcohols, such as heptanol, as the first extraction solvent to obtain a glucose-rich raffinate phase and an extract phase rich in the acid and the first extraction solvent; extracting the extract phase rich in the acid and the first extractant for the second time with a second extraction solvent, such as benzene, carbon tetrachloride or toluene, to obtain a raffinate phase containing water and sulfuric acid and an extract phase containing the first and the second extraction solvents; separating the first and the second extraction solvents by distillation with result that the recovered sulfuric acid, the first and the second extraction solvents are reusable; and finally neutralizing the small amount of residual sulfuric acid still contained in the glucose-rich phase with lime and then filtrating to obtain the glucose-rich phase without the sulfuric acid. According to the preferred embodiment of this invention, the hydrolysate obtained from the post hydrolysis process contains 55% sulfuric acid, 40.5% water and 4.5% sugar, so the radio of the sulfuric acid to the sugar equates to 12.2. The extract phase from the first extraction contains 79.4% heptanol, 14.5% sulfuric acid, 5.3% water and a minute amount of sugar, so the radio of the heptanol to the sulfuric acid equates 5.5. In the second extraction, the radio of benzene to heptanol equates 5. If 1 kg glucose needs to be extracted, the amount of benzene to be distilled is: 12.2×5.5×5=335.5 kg, and the burning energy is: 335.5 kg×434 kJ/kg=145607 kJ. However the energy produced by oxygenolysis of the 1 kg glucose is only 15945 kJ, which is far less than the burning energy for distillation the benzene. Obviously it is a process in which the output energy is far less than the input energy, therefore has no practical value. What's more, this process utilizes organic solvent as the second extraction solvent to separate the alcohol and the acid, so a large amount of the second extraction solvent needs to be recovered which, in turn, lead to high energy consumption.
Obviously, in the conventional process, the high cost of recovering sulfuric acid limits the concentrated sulfuric acid hydrolysis method to be used widely.